NOT of an integer in C

Question

#include <stdio.h>
int main()
{
    int i;

    i=1;
    printf("%d ",!i);

    i=5;
    printf("%d ",!i);

    i=0;
    printf("%d\n",!i);

    return 0;
}

I got the following output in C: 0 0 1

What is the logic behind the output?

http://stackoverflow.com/questions/2319766/how-does-logical-negation-work-in-c — Suvarna Pattayil, Jun 11 '13 at 15:17

score 7 · Answer 1 · answered Jun 11 '13 at 15:17

7

In C, any non zero value is considered to be a true value. So taking the logical negation with ! converts it to 0. The logical negation of 0 is 1.

answered Jun 11 '13 at 15:17

jxh

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score 2 · Answer 2 · answered Jun 11 '13 at 15:18

2

In C booleans are integers where 0 is false and any other value is true.

! is NOT (as you know) so it turns any value that is not 0 into 0 and it turns 0 into 1.

answered Jun 11 '13 at 15:18

Simon

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score 1 · Answer 3 · edited Jun 11 '13 at 15:21

1

i is used like a boolean value:

If i != 0, then !i == 0.
If i == 0, then !i == 1.

edited Jun 11 '13 at 15:21

Lynn

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answered Jun 11 '13 at 15:16

Claudio

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score 1 · Answer 4 · answered Jun 11 '13 at 15:19

What do you mean by "logic"?

The specific behavior of ! operator? It is defined by the language standard. It produces 0 for non-zero argument. And 1 for zero argument. That's the way it is defined.

The rationale behind such definition? Well, it is supposed to implement the logical-not behavior. Historically, in C language logical "false" is represented by zero integer values, while everything non-zero is interpreted as logical "true". So, that's what you observe in your experiment. When ! operator (or any other logical operator in C) has to generate a "true" result, it uses 1 to represent it, not just some arbitrary non-zero value.

score 0 · Answer 5 · answered Jun 11 '13 at 15:17

0

You are performing a Boolean operation. The '!' is NOT an inverter as one would normally think of it. If you are looking for the inverter, use the '~'.

answered Jun 11 '13 at 15:17

Lee

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score 0 · Answer 6 · answered Jun 11 '13 at 15:19

! is a boolean operator that inverts the given input, from true to false and false to true. True is anything that is not zero. False is zero. So, when you notted 1 or 5, you invert a true value, which prints the integer value of false, 0. Next when you invert a false value, it prints the integer value of true (default 1)

NOT of an integer in C

6 Answers6